1. Field of the Invention
Aspects of the present invention relate to a cathode active material, a method of preparing the cathode active material, a cathode containing the cathode active material, and a lithium battery containing the cathode active material. More particularly, aspects of the present invention relate to a cathode active material containing a metal oxynitride or metal nitride, a method of preparing the cathode active material, a cathode containing the cathode active material, and a lithium battery containing the cathode active material.
2. Description of the Related Art
Li—Co based oxides, which are used in secondary batteries, have a high conductivity and high energy density and provide a long operation time. For example, LiCoO2 has a conductivity of about 10−4 to 10−3 S/cm. However, LiCoO2 has an electrical capacity of only about 140 mAh/g and is expensive. When a battery is charged, some of the lithium is removed from LiCoO2 to form Li1−xCoO2 (0<x<1), which is inherently unstable in a battery.
Lithium metal phosphate is inherently stable. For example, LiFePO4 has a strong bonding of Fe—P—O, and thus, it is difficult to liberate oxygen even when a battery is short-circuited or over-heated. The theoretical electrical capacity of LiFePO4 is about 170 mAh/g, but its substantial electrical capacity is about 110 mAh/g due to its low conductivity.
To improve conductivity of LiFePO4, LiFePO4 can be doped with a transition metal or an alkali metal, or can be coated with carbon. JP P2003-292308 discloses a technique of coating a carbonaceous material on LiFeO4 particles. The coating results in an increase in conductivity from 10−9 S/cm to about 10−7 to 10−5 S/cm, and the carbon-coated LiFeO4 has a substantial electrical capacity of about 165 mAh/g.